A mammography unit is an X-ray system for examining a breast of a patient. Before beginning to take an X-ray, the object being examined is placed on an object table, belonging to a mammography unit and having an X-ray receiver, and is compressed by a compression plate. Typically, the compression plate is disposed on an adjustable-height compression plate mounting of the mammography unit by means of a releasable mounting mechanism, which makes it possible for the particular compression plate to be exchanged for a compression plate intended for the particular examination method or the particular object being examined. A mammography unit of this kind is disclosed for instance in U.S. Pat. No. 5,170,419 A.
For performing a biopsy of the particular object being examined, special compression plates are provided, with an opening that allows access to the particular region to be examined of the object being examined. For position marking a puncture point for the biopsy, a shadow cross device is known that can be disposed in front of an X-radiation diaphragm of the mammography unit and in a light projection of a light visor makes a particular position detectable by means of a cross-shaped shadow. The particular position marked is adjustable manually relative to a scale located beside the opening on the compression plate. This kind of shadow cross device can be procured as accessory equipment from Siemens AG (Munich, Germany), as an example.
For position marking, instead of the shadow cross device, the laser device known as “Laser Positioner II” made by Livingston Products Inc (Buffalo Grove, Ill.). can also be disposed on the X-radiation diaphragm; this laser device marks the particular position by projecting a position marking in the shape of a cross. Since the laser emitter of the laser device is offset from the X-radiation, a parallax error can occur, depending on the spacing between the laser device and the object being examined; this error is corrected by an automatic correction of the projection angle of the laser emitter.
For X-ray images that are intended to perform a biopsy, the laser device is positioned at a distance from the X-radiation diaphragm, since it limits the X-radiation field and hinders the view of the object being examined and makes access to the object being examined more difficult. The power supply is provided via a battery located in the laser device; in this way, the need for making a power connection with the mammography unit in addition to attaching the laser device is avoided.
FIG. 1 shows a mammography unit 2 in accordance with the related art; including a support member 15, a mounting arm 16 located on the support member 15; an X-ray emitter 10 for generating X-radiation 13, disposed on the upper end of the mounting arm 16; an X-radiation diaphragm 11 for delimiting the X-radiation 13 onto an intended X-radiation field, an adjustable-height compression plate 5; and, an X-ray receiver 14 located on the lower end of the mounting arm 16. In FIG. 1, the contour of the maximum radiation field of the X-radiation 13 is shown. The X-radiation 13 penetrates a particular object being examined 4 before striking the X-ray receiver 14 that generates an X-ray image of the object being examined 4. The object being examined 4 is compressed by the compression plate 5, which includes both a mounting frame 7 and a transparent compression member 6. The compression plate is releasably connected to a compression plate mounting 8 that is located on a guide 9 of the mammography unit 2, and is adjustable in height.
A shadow cross device 12, with which a position marking in the shape of a shadow cross can be projected onto the object being examined 4, is located on the X-radiation diaphragm 1. As a result, for instance in a biopsy, a particular intended puncture point on the object being examined 4 can be marked. The shadow cross devices 12 like all the other devices that can be disposed on the diaphragm 11 of the mammography unit 2 for projecting a position marking, restrict the X-radiation field to only a fraction of the maximum possible X-radiation field. To make a full-field image of the object being examined 4 with the mammography unit 2, the shadow cross device 12 has to be removed from the diaphragm 11.